The answer is thoroughly covered by @wyfwa4. You should accept that one, and upvote this one.
Here's the tl;dr -
A base search always runs in fast mode, which optimizes out everything it doesn't need. It cannot see the post process searches, and pays no attention to them. If you don't tell the base search which fields to keep, the base search will throw them all away.
Add a table or fields command at the end of the base search to keep each field you want to use in any of the post-process searches.
| fields host instance counter Value
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